In general, an antireflection film is disposed on the outermost surface of image display devices such as cathode-ray tube display devices (CRT), plasma display panels (PDP), electroluminescent display devices (ELD) and liquid crystal display devices (LCD). This is for preventing contrast reduction or image reflection owing to external light reflection on the displays, generally having a function of reflectivity reduction owing to light scattering on surface protrusions or to light interference of multi-layered thin films.
The antireflection film is generally produced by forming, on a transparent support, a film of a low-refractive index layer having a suitable thickness and having a lower refractive index than the transparent support. For realizing its low refractive index, the material for the low-refractive index layer is desired to have a refractive index as low as possible. Since the antireflection film is disposed on the outermost surface of displays, it is desired to have high scratch resistance. In order to realize high scratch resistance of thin films having a thickness of 100 nm or so, the films must have high mechanical strength and must be adhesive to underlying layers.
The antireflection layer of an antireflection film formed of multi-layered thin films generally has a layer constitution consisting essentially of a high-refractive index layer and a low-refractive index layer for enhancing the antireflectivity of the film. For attaining efficient antireflection, it is said that the refractive index difference between the high-refractive index layer and the low-refractive index layer must fall within a specific range (see, for example, JP-A-59-50401). For this, various methods have been investigated. MgF2 and silica are used as low-refractive materials for the low-refractive index layer (see, for example, JP-A-2-245702); a fluorine-containing compound is used (see, for example, JP-A-2003-121606); or 2 or more inorganic particles are piled up to form micro-voids (see, for example, JP-A-11-6902). In fact, however, all these methods are problematic in that the materials are difficult to obtain and are unstable and their films are not strong and are not resistant to staining, and sufficiently satisfactory low-refractive index materials could not as yet obtained.
For increasing the mechanical strength of films in some degree so as to make them have increased scratch resistance, a method of using fluorine sol-gel films is proposed in JP-A-2002-265866 and JP-A-2002-317152. However, this has significant limitations in that (1) curing the films requires long-time heating and the production load is therefore great, and (2) the films are not resistant to saponification solution (alkali-processing solution), and saponification of the surface of transparent plastic film substrates after the formation of antireflection film thereon could not be carried out. In addition, the films are not satisfactorily resistant to staining. For satisfying all of low-refractive index and good scratch resistance and good stain resistance, low-surface free energy compounds that have a low refractive index and have a high mechanical strength and are hardly stained on their surface are needed. However, most of such compounds are often in a trade-off relation, or that is, when one of the requirements is improved, then the other is worsened. Accordingly, it is difficult to satisfy all the requirements of refractive index reduction and improvement of scratch resistance and stain resistance.
For the intended refractive index reduction, JP-A 7-287102 discloses a technique of increasing the refractive index of a hard-coat layer to thereby reduce the refractive index of the antireflection film. However, the high-refractive index hard-coat layer causes film color mottles since the refractive index difference between the layer and the support is large, and the wavelength dependency of the refractive index of the film greatly increases.
On the other hand, in JP-A-11-189621, JP-A-11-228631 and JP-A-2000-313709, there is proposed a method of improving the scratch resistance of films by introducing a polysiloxane structure into a fluorine-containing polymer so as to lower the friction coefficient of the film surface. The method may be effective in some degree for the improvement of scratch resistance of the films, but is still unsatisfactory in that films essentially not having high strength and high interfacial adhesiveness could not be improved to have sufficient scratch resistance by the method.
JP-2003-292831 discloses a low-refractive index coating agent and an antireflection film. In this, the agent comprises at least an active energy ray-curable resin that contains a compound having a (meth)acryloyloxy group in the molecule, and hollow particles having a mean particle size of from 0.5 to 200 nm. However, this is also problematic in that, when such a thin-film system is cured in an atmosphere in which the oxygen concentration is near to that in air (oxygen concentration: about 20% by volume), the reactive group in the film could not well react owing to curing failure and therefore the scratch resistance of the cured film is not good. In addition, since the low-refractive index layer in JP-2003-292831 is not processed to be resistant to staining, this is still problematic in that stains such as finger marks or water marks may readily adhere to the films and the stains adhered to them are difficult to wipe away. These days in particular, applications such as TVs and monitors are required to have good scratch resistance and good stain resistance.
In JP-A-7-133105 and JP-A-2001-233611, hollow silica particles are proposed as a low-refractive index material. These are excellent as a low-refractive index material, but it has been found that, when they are used in a low-refractive index layer of antireflection films, their adhesiveness is poor and they detract from the commercial value of the outermost film of displays, and therefore, their amount to be used must be limited.
JP-A-7-333404 describes an antiglare antireflection film having good gas-barrier properties, antiglare properties and antireflection properties. However, since this requires a silicone oxide film to be formed in a mode of CVD, its producibility is low when compared with wet-coated films.
The recent tendency in the art is toward large panels in image display devices (e.g., LCD, PDP, CRT), especially toward liquid crystal display devices with an antiglare antireflection film disposed therein. For protecting such expensive large-panel image display devices, it is desired to further improve the protective films for antiglare films and antireflection films for them. Concretely, the films are required to have better visibility (with neither glare look nor external light reflection in display images, with high image sharpness and transparency), the display panels are not soiled by finger marks or by dust adhesion thereto, and they are highly resistant to scratching. In liquid crystal display devices (LCD), a polarizing plate is an indispensable optical material. The polarizing plate generally comprises a polarizing sheet protected by two protective films. For reducing the number of the constitutive members of liquid crystal display devices and for increasing the producibility thereof with reducing the production costs thereof, it is desired to make the protective films for the polarizing plate have an antireflection function so as to make the resulting polarizing plate enjoy better weather resistance, physical protection and antireflection. This is especially for realizing the increase in producibility, the cost reduction and the thickness reduction of the devices.
As a material of the polarizing sheet, polyvinyl alcohol (hereinafter referred to as PVA) is principally used. Briefly, a PVA film is monoaxially stretched, then colored with iodine or a dichroic dye, or it is first dyed and then stretched, and this is crosslinked with a curable compound to form a polarizing sheet. In general, the polarizing sheet is stretched in the running direction (machine direction) of a long film (stretching along machine direction), and therefore, the absorption axis of the polarizing sheet is nearly parallel to the machine direction.
The protective film is an optically-transparent film having a small birefringence, for which cellulose triacetate is mainly used. Heretofore, the protective film is stuck to the polarizing sheet in such a manner that the slow axis of the protective film could be perpendicular to the transmission axis of the polarizing sheet (or that is, in such a manner that the slow axis of the protective film could be parallel to the absorption axis of the polarizing sheet).
antireflection treatment is attained by the use of an antireflection film formed as a multi-layered product of multiple thin films of different materials having a different refractive index, and the antireflection film is so planned that it could reduce the reflection in a range of visible light. However, in the antireflection film, the thickness of the constitutive layers of the multi-layered product is constant in each layer, and therefore, in principle, the antireflection film could not completely attain its function of antireflection in the entire region of visible light.
Accordingly, in general, the antireflection film is so planned that its antireflection to light of high visibility at around 550 nm is emphasized and that its antireflection could cover a wavelength range as broad as possible. For the reasons in planning the film, the antireflection effect of the film is at present not sufficient in the other ranges than the specific wavelength range, and the reflectivity of the film in a part of a short wavelength region of visible light and in a part of a long wavelength region thereof is larger than that in the other wavelength region of the visible light region. As a result, this is problematic in that the reflected light exhibits a specific color hue and it detracts from the display quality of devices.
As opposed to it, a different investigation has been made of integrating a polarizing plate and an antireflection film to construct an integrated and stuck product so as increase the quality of display images. Concretely disclosed is an antireflection film-combined polarizing plate having a reflectivity of at most 3.5% in a range of from 380 to 700 nm (see, for example, JP-A-2003-270441).
In reflection-type or semi-transmission reflection-type liquid crystal display devices, the backlight used generally has a bright line peak at three wavelengths of 440 nm, 550 nm and 610 nm. Therefore, it is an important point to make the transmittance at these three wavelengths the same for improving the color reproducibility of the devices. An antireflection film-combined polarizing plate is proposed, of which the parallel transmittance and the cross transmittance at wavelength 440 nm, 550 nm and 610 nm are specifically defined (JP-A-2002-22952 and JP-A-2003-344656).
On the other hand, for improving display quality of devices, various optical layers in addition to polarizing plate are provided on the outermost surface of liquid crystal panels. For example, retardation plates (including λ plates such as ½ wavelength plate and ¼ wavelength plate), optical compensation films and brightness-improving films are employed. In particular, elliptically-polarizing plates or circularly-polarizing plates constructed by superposing a retardation plate on a polarizing plate that comprises a polarizing sheet and a protective layer; polarizing plates constructed by superposing an optical compensation film on a polarizing plate that comprises a polarizing sheet and a protective layer; and polarizing plates constructed by superposing a brightness-improving film on a polarizing plate that comprises a polarizing sheet and a protective layer are put into practical use.
The polarizing plate constructed by superposing a brightness-improving film on a polarizing plate that comprises a polarizing sheet and a protective layer is generally provided on the back side (lower side) of a liquid crystal cell, and it acts to increase the brightness of the image display panel of LCD (see, for example, “2003's Views and Strategies in High-Function Film Market”, p. 91 (2003), by Yano Keizai Kenkyu-jo).
On the other hand, an antiglare and/or reflection reduction-processed film is proposed for the visible-side protective film of a polarizing plate that is to be disposed on the visible side (upper side) of a liquid crystal cell (see, for example, JP-A-2003-149634).